Effect of magnetic defects and dimensionality on the spin dynamics of GeMn systems: Electron spin resonance measurements

Abstract

Effects of dimensionality on magnetic and electric properties of one- and two-dimensional GeMn systems and the role of defects in magnetic ordering are investigated by means of electron spin resonance (ESR) and superconducting quantum interference device magnetometry techniques. Arrays of ${\text{Ge}}_{1\ensuremath{-}x}{\text{Mn}}_{x}$ nanowires and thin Ge:Mn films with similar concentrations of the magnetic impurity $(x=1%--8%)$ have been fabricated by chemical synthesis and ion implantation, respectively. In magnetically homogeneous ${\text{Ge}}_{1\ensuremath{-}x}{\text{Mn}}_{x}$ nanowires, all observed electron spin resonances are related to absorption on individual magnetic centers (${\text{Mn}}^{3+}$ and ${\text{Mn}}^{2+}$ ions and polarized charge carriers) in a broad temperature range, $T=5--300\text{ }\text{K}$. On the other hand, in strongly inhomogeneous 2D GeMn films, a collective spin excitation, the spin-wave resonance, is observed at low temperatures, $T=5--60\text{ }\text{K}$. This signifies the presence of long-range spin states and a cooperative magnetic response originating from crystalline ${\text{Mn}}_{5}{\text{Ge}}_{3}$ precipitates and Mn-rich amorphous nanoclusters as well as diluted Mn ions. Additionally, a strong negative background was observed and attributed to the microwave magnetoresistance of the Ge:Mn thin films. The absence of the magnetoresistance in ${\text{Ge}}_{1\ensuremath{-}x}{\text{Mn}}_{x}$ nanowires indicates that the scattering of charge carriers is determined by dimensions of the structure. Overall, our analysis of magnetic-resonance phenomena reveals a significant difference between one-dimensional and two-dimensional magnetic semiconductors. It emphasizes the important role of dimensionality as well as the type and distribution of magnetic defects in spin-dependent scattering and dynamic magnetic properties of GeMn semiconductors.